snapshot: Fix issue with snapshot rollbacks

Closes: #2144.

- '--ambit' seems to a required argument if there is no default subvolume set on
the filesystem. Add it to prevent error during rollback.

- Description is not a required option for rollback (anymore?) and default
descriptions for the two snapshots are more descriptive.

Tests:

- On a fresh vagrant machine, run snapshot rollback with the patch. It fails.
With the patch, rollback succeeds.

- The description created for the rollback is the default one 'rollback backup'
and 'writable copy of #x'.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: James Valleroy <jvalleroy@mailbox.org>

plinth/modules/snapshot/privileged.py

@@ -251,7 +251,12 @@ def kill_daemon():
 @privileged
 def rollback(number: str):
     """Rollback to snapshot."""
-    command = [
-        'snapper', 'rollback', '--description', 'created by rollback', number
-    ]
+    # "ambit" is not very well documented by snapper. Default ambit is "auto"
+    # which errors out if default subvolume is not yet set on the filesystem.
+    # If set, then it acts as "transactional" ambit if the default snapshot is
+    # readonly, otherwise it acts as "classic". The "classic" behavior is the
+    # one described snapper(8) man page for rollback behavior. The classic
+    # behavior when a snapshot number to rollback to is provided is the
+    # behavior that we desire.
+    command = ['snapper', '--ambit', 'classic', 'rollback', number]
     subprocess.run(command, check=True)